The present invention is in the field of imaging techniques, and relates to a method and system for spectral imaging.
Spectral imaging is aimed at providing at least some spectral information about an object at every location in an image plane. Various spectral imaging techniques have been developed, including multispectral imaging, hyperspectral imaging, full spectral imaging, imaging spectroscopy or chemical imaging. Spectral images are often represented as an image cube, a type of data cube.
Multispectral (MS) and hyperspectral (HS) cubes could be acquired in many ways. Some systems (utilizing whiskbroom, pushbroom and tunable filters for realizing HS imagers), rely on multiple acquisitions of 1D or 2D subsets of the 3D HS cube followed by simple reconstruction. Some other systems include polychromatic sensors that trade-off resolution with spectral information (similar to the Bayer CFA) and require spatio-spectral reconstruction algorithms [Y. Monno, M. Tanaka and M. Okutomi Proceedings of IS&T/SPIE Electronic Imaging (EI2012), Digital Photography VIII, Vol. 8299, pp. 82990O-1-7, January, 2012; Y. Murakami, M. Yamaguchi, and N. Ohyama, “Hybrid-resolution multispectral imaging using color filter array,” Opt. Express 20, 7173-7183 (2012)].
Recently, several HS snapshot acquisition techniques have been developed. Some of them are based on compressed sensing in which the HS image is assumed to be sparse, and an additional optical element is used within the imaging system, to compress the data. Such techniques are described in the following publications: A. Stern, “Compressed imaging system with linear sensors,” Opt. Lett. 32, 3077-3079 (2007); A. Wagadarikar, R. John, R. Willett, and D. Brady, “Single disperser design for coded aperture snapshot spectral imaging,” Appl. Opt. 47, B44-B51 (2008); C. Li, T. Sun, K. F. Kelly and Y. Zhang. A compressive sensing and unmixing scheme for hyperspectral data processing. IEEE_J_IP 21(3), pp. 1200-1210. 2012; M. A. Golub, M. Nathan, A. Averbuch, E. Lavi, V. A. Zheludev, and A. Schclar, “Spectral multiplexing method for digital snapshot spectral imaging,” Appl. Opt. 48, 1520-1526 (2009).
However, these techniques require prior knowledge of the scene being imaged, and also typically suffer from low light efficiency, and systems implementing such techniques are rather complex.
As for the integral field spectroscopic systems, the common underlying principle of these systems is similar to light field cameras in the sense that the spectral information is traded-off with spatial resolution. Thus, a number of spectral bands in the detected light is equal to the resolution degradation ratio. Integral field hyperspectral imaging techniques, such as lenslet array, fibre array, image slicer and micro-slicer, all exhibit this behavior. Yet another known solution concerns the use of a 2D grating that diverges incident light according to the grating' diffraction order to form multiple, multispectral sub-images on the sensor; this is followed by reconstruction algorithms. This method allows fast hyperspectral cube acquisition, but the resultant image suffers from low spatial resolution; also the required setup could not be integrated in common cameras.
General Description
The present invention provides a novel technique for hyperspectral imaging that enables to acquire a complete hyperspectral cube of arbitrary scene. The invention provides for acquiring a hyperspectral cube with full spatial resolution and spectral resolution. This technique needs neither any preliminary information of the scene nor additional assumptions.
A hyperspectral imaging system of the invention includes a wide spectral filter/etalon, e.g. a clear aperture Fabry-Perot etalon with wide transmission peak, placed in front of a pixel matrix of a detector, i.e. upstream of the pixel matrix with respect to the input light propagation direction. The system operates in a “multiple-exposure” mode to acquire multiple frames of the scene, while using multiple different, partially overlapping transmission curves of the etalon. More specifically, the system operates to acquire a set of frames, each with a different transmission function of the etalon, i.e. slightly displaced transmission peak of the etalon. The transmission profiles are wide to capture more light and consequently improve the signal to noise ratio. Thus, each frame is acquired with a pre-determined weighted sum of wavelengths, and the spectral profiles of each two exposures (frames) may significantly overlap. Following the acquisition, spectral reconstruction algorithms are applied to recover the spectrum of the image. In general, the number of spectral bands is equal to the number of exposures.
Thus, the present invention provides for acquiring a complete hyperspectral cube of arbitrary scene with full spatial and spectral resolutions, while having a simple construction of standard imaging system, additionally equipped with a tunable dispersive element (spectral filter) with broad spectral transmission profiles that allows acquiring within each frame a weighted sum of wavelengths. The hyperspectral imaging system of the invention could be integrated within both color (Bayer) and monochrome image sensors. The clear aperture Fabry-Perot etalon may also be used for standard color imaging, thus enabling a dual-purpose imaging system (standard color+hyperspectral). The system has improved noise performance, since each frame is acquired with reduced noise content.
The approach of the invention is general and may fit multiple imaging scenarios, including UV/VIS/NIR; and is not sensor dependent. The imaging system of the invention may be integrated in industrial cameras, surveillance cameras, medical devices, quality control equipment, spectrometry systems for inspecting chemical compounds and biological tissues.
The present invention takes advantage of the hyperspectral imaging technique described in U.S. patent application No. 62/075,972, assigned to the assignee of the present application, and incorporated herein by reference. This technique is based on applying angular coding on an input light field while creating an optical image thereof, thus providing angular multiplexing of hyperspectral image data (hyperspectral cube). This technique is effectively operable for objects of a size that can be imaged on at least N pixels of the pixel array to allow reconstruction of N spectral bands.
The technique of the present invention provides for effective spectrum reconstruction of very small objects up to single-pixel objects. To this end, the invention utilizes multi-exposure approach with different transmission functions of the wide spectral etalon.
The tunable etalon is configured for tunability over a relatively wide spectral range, and also preferably with sufficient Free Spectral Range (FSR). Furthermore, in order to enable generation of accurate color images, the etalon is preferably configured with low finesse (namely sufficiently wide spectral transmission peak). This, on the one hand, provides for creating images with accurate (e.g. faithful) colors, and, on the other hand, allows sufficient light to pass to the sensor (pixel matrix).
The etalon typically includes a pair of substantially parallel, generally reflective surfaces, spaced from one another by a gap (optical distance). Generally, the transmission function T of such etalon is a function of wavelength λ, transmission of the two surfaces, and the value of gap between these surfaces. For the etalon formed by given reflective surfaces (i.e. given transmission(s) thereof), the etalon's transmission is a function of wavelength and gap, T(λ, gap). Thus, by controllably varying the gap between the reflective surface to provide a sequence of n different gap values during image acquisitions of n frames (n exposures), respectively, different n transmission functions of the etalon are applied to the input light field which differently affect detection of light components of n wavelengths λ1, . . . λN by the pixel matrix of the detector. This allows reconstruction of the spectrum of the object by processing the image data of n frames.
Thus, according to one broad aspect of the invention, there is provided a hyperspectral imaging system for use in reconstructing spectral data of an object, the system comprising:
a pixel matrix of a detector;
a dispersive unit in front of the pixel matrix; and
a control system comprising: a controller for tuning the dispersive unit to provide n different partially overlapping spectral transmission profiles thereof during n image acquisition sessions; and a control unit in data communication with the detector and being configured and operable for processing n image data pieces generated by the pixel matrix in said n image acquisition sessions respectively, each being indicative of a spectral image detected by the pixel matrix and corresponding to the different spectral transmission profile of the dispersive unit; and determining the reconstructed spectral data of the object.
The transmission profile of the dispersive unit is a function of wavelength and a tunable parameter (e.g. gap) of the dispersive unit. The plurality of the image data pieces is a function of the plurality of the transmission profiles of the dispersive unit and the spectrum of the object to be determined.
The dispersive unit comprises a spectral filter, e.g. an etalon, configured with a wide spectral range. Passage of light through the dispersive unit provides wavelength multiplexing of image data at the pixel matrix, such that detected light intensity at the pixel corresponds to the spectral data of the image multiplexed with the transmittance function (dispersion profile/pattern) of the dispersive unit.
The controller, configured and operable for tuning the dispersive unit to sequentially produce the different transmittance profiles for acquiring the sequential frames of the object, may be integrated with the control unit associated with the detector or may be a separate utility.
According to another aspect, the invention provides an imaging method for use in reconstructing spectral data of an object. The method comprises: sequentially acquiring a plurality of n image frames, by performing n imaging sessions of the object onto a pixel matrix of a detector, while sequentially applying to the light being imaged n predetermined dispersion profiles being different from one another and partially overlapping, thereby obtaining n image data pieces indicative of n different spectral images of the object; and processing said n image data pieces utilizing the data about the predetermined n dispersion profiles, and reconstructing n spectral bands of the object being imaged.
The application of the dispersion profile to the light is achieved by interacting the light with a dispersive pattern. The image data piece is a function of the corresponding dispersive profile and a spectrum of the object to be determined.
The invention in its further aspect provides a control unit for reconstructing spectral data of an object. The control unit is configured for receiving and processing input image data (either directly from an imaging system or from a storage device). The received image data comprises data indicative of n image data pieces corresponding to n spectral images obtained by a pixel matrix, where each of the n spectral images is formed by light coded by a different dispersive profile. The control unit comprises an analyzer configured and operable for utilizing data indicative of the n dispersive profiles in association with the respective n image data pieces and determining the spectral data of the object.